The present invention relates to an electrically supported power steering system for motor vehicles. The power steering system includes an input shaft mechanically linked to a steering wheel and used to transmit a torque required for the steering of wheels to be steered. An output element is mechanically linked to the wheels. A servo motor through which servo power can be applied directly or indirectly to the output element is arranged on the power steering system. The input shaft and the output element are connected to one another via a rotation-elastic element so that a limited torsion movement is possible between the input shaft and the output element. A contactless detection unit is used for contactless detection of the direction and intensity of a steering torque acting on the input shaft and of other steering-specific parameters. The detection unit includes at least one sensor respectively connected to the input shaft or the output element.
Such a power steering system is described, for example, in German Published Patent Application No. 38 44 578. In this power steering system, two drums made of non-magnetic material are respectively connected to an input shaft or an output element. The drums are provided with magnetizable media on their periphery or on an end face, so that they form alternating magnetic north and south poles. By measuring the difference in the rotation angles of the two drums, the torque is determined using magnetoresistive elements.
The servo support of an electric steering system can be controlled by detecting the torque. If the servo support of an electric steering system is controlled only as a function of the torque, this is perceived as unusual in a driving operation, because the friction conditions in that case are different from those of a hydraulically supported power steering system. There is no xe2x80x9cfeedbackxe2x80x9d of the steering rate from the steering torque. In order to compensate for this disadvantage, the steering rate must be detected. In order to detect the steering rate, an additional sensor is provided in conventional electric steering systems, for example, in German Published Patent 37 11 854. This sensor has a DC tacho-generator, for example, which generates a direct current having a voltage adjusted to the steering rate. Such a steering rate sensor involves a relatively high cost for detecting the steering rate.
It is an object of the present invention to improve the control of an electrically supported power steering system. In particular, it should allow steering-specific parameters to be detected inexpensively and in a simple manner.
The above and other beneficial objects of the present invention are achieved providing a power steering system in which a detection unit of the generic power steering system contains a single pulse generator designed as a magnet ring, which is provided with alternating magnetic north and south poles on its peripheral surface or on its end face, so that both the relative position of the input shaft and the output element with respect to one another, as well as the change in the position of the input shaft, are detected via allocated sensors. This design results in low assembly costs. In addition, the manufacturing costs of the steering system are reduced due to the small number of components.
Thus, the electrical connection between rotating parts of the sensors and a stationary sensor housing can be established via a flat spiral spring. This allows the signals to be transmitted in a simple and reliable manner.
Instead of a flat spiral spring, a wiper unit may also be used for the electrical connection between rotating parts of the sensors and a stationary sensor housing.
In order to also be able to detect the absolute steering angle in a simple manner, a gear, through which a plurality of revolutions of the input shaft or of the output element can be converted into small rotation or translation movements, is arranged on the output element. The gear contains a thread connected to the output element or the input shaft and a threaded nut, the threaded nut being guided by an axial guide in an axially movable but non-rotatable manner. In addition, the threaded nut is connected to a magnet that generates a signal with its axial movement for the absolute steering angle.
Using such a gear, a magnetoresistive sensor, which is used for detecting steering-specific parameters such as steering angle, steering rate, and steering acceleration, and another magnetoresistive sensor, which is used for detecting the absolute steering angle, are arranged on a common circuit board and work together with a common electronic unit.
The sensor units for detecting the steering-specific parameters torque, steering direction, steering angle, absolute steering angle, steering rate, and steering acceleration may be combined in a single unit. This in turn results in low assembly costs.
When using an electric motor as a servo motor, the measured parameters may be used for controlling the motor speed in a simple manner, resulting in better control quality.